Aft CG Testing

[jaredyates]

Has anyone loaded a 4-place Bearhawk to 22.5" and 2500 pounds yet? I progressively worked my way back to 21.88" today and didn't care to go any further aft. I figure that with the way I'm measuring and calculating, 21.5 is my new operational limit. I had no trouble getting the airplane back on the ground safely, but flying it was not what I would call fun. For example, from a level, trimmed 80 knots, I displaced the stick 2" aft and let go. The resulting pitch up recorded 1.6g on my Dynon and continued to climb to about 20 degrees of pitch before I intervened. At that pitch, the speed had dropped enough that it was going to start pitching down soon, so it was still self-correcting and thus stable, just not very much. Bob said to me a few months back that I wouldn't like flying at the aft CG range, and he was right. As usual.

[DesertBearhawk]

As we say...nose heavy flys like crap, tail heavy flys once....

[Enewton57]

Here is a write-up of my CG testing with the O-360 Lycoming installed like you have Jared:

http://mybearhawk.com/flying/25hours.html

I never got any further aft than 20" where it flew fine, but I remember I was starting to feel like I didn't want to go any further aft with it with my limited pilot experience.

[Battson]

Yeah we tested it, at a little less than 2700lbs and CG right at the aft limit. It flies much the same at 2500lbs aft limit. Main difference at full aft MTOW (apart from being heavy) is it doesn't need much trim for landing, and of course the stalls feel different, but as ever - the Bearhawk showed no bad habits.

The more I think about it, some other points -
You won't be able to raise the tail on take-off until about 25kts or so. It's going to feel different on climb out. Normally neutral trim is about right, but with full aft and heavy you're going to need to pole forward / trim forward once you're up to climb speed. Steering on the ground during taxi is painful, compared to when it's light in the tail - but with power on it's no worries. Also on roll out, you can easily keep it straight if you're paying any attention to the task at hand. I like to keep the tail up with brakes and elevator, but with a heavy tail of course it comes down faster, and if you land tail-low then you can't easily raise the tail. Steering on the rollout is fine with the tail down, even without tailwheel steering - just use brakes as required.

Stall speed was about 42kts indicated.
Climb with IO-540 was about 800ft/min.
Take-off was about 177m ground roll.
Aircraft was still positively stable in all flight regimes.
Ground handling is the biggest loss of "performance".

[Erbman]

I haven't kept records of my cg positions, but I'm thinking that the most aft I have ever flown with is around 21.5 inches. That would either be packed up for Oshkosh or when I had 3 adults and a 7-year old in the airplane. Generally the only problem I have had is a tendency to PIO on takeoff transitioning to the climb. I compensate for this by trying to be very smooth on my pitch inputs so that I don't have to rapidly change directions of inputs.

I haven't flown with any more aft cg because I haven't had an operational need to. Even so, I'm convinced that Bob's stated aft cg limit is reasonable.

I did ask Bob about ways to improve my cg envelope. He suggested that changing from the flat horizontal tail to the airfoil shaped horizontal tail would make the stabilizer more effective and possibly increase the cg range, though he couldn't say how much. I haven't done this yet because it would require recovering and repainting the tail. If I chose to do that, I would want to do some flight tests first to measure where the aft cg limit is right now so that I would be able to tell if it had actually moved.

[Battson]

In terms of increasing the CG range --- assuming the aft CG-limit's location is determined by the need to keep the CG forward of the CP of the wing, I don't think you could extend the CG aft limit without losing pitch stability.
I don't know, but, it might be possible to extend the forward CG limit with the new stabiliser, as that limit is often related to horisontal stabiliser & control effectiveness? Perhaps that is what Bob was talking about.

[Erbman]

The issue is not keeping the CG ahead of the center of pressure of the wing. The cg needs to be ahead of the neutral point. The position of the neutral point depends on both the wing and the horizontal stabilizer. Making the horizontal stabilizer more effective (or just bigger) will move the neutral point aft.

[Battson]

I guessed there must have been a name for the practical aft limit, because the main influences on stability (CG distance to CP) can't approach zero without becoming unstable.

It's not clear to me how much the neutral point could move aft with more tail force. It must already be quite close to the CP already on the Bearhawk.